Method of preparing lightweight window anti-static circuit and optional heating circuit

ABSTRACT

A method of producing a transparent plastic window containing embedded therein a novel static electricity dissipating circuit (hereinafter referred to as an anti-static circuit), which may also contain a heating circuit insulated therefrom.

This is a division of Ser. No. 700,239 filed on June 28, 1976 now U.S.Pat. No. 4,078,107.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to transparent heated windows having an outersurface of hard plastic suitable for use in lightweight aircraft. In thepast, laminated windows for aircraft have been provided with internallylocated heating circuits that are energized to remove fog, mist or icethat deposits on the outer surface of the window in flight andelectroconductive anti-static coatings on the outer surface connected toground to dissipate any charge of static electricity that developsduring operation of an airplane before the static charge is dissipatedthrough the heating circuit by forcing an electrical path through thewindow thickness from the outer surface where the static charge developsto the heating circuit.

One type of such window is composed of laminated glass comprisingalternate plies of rigid transparent dielectric material, such as glassor a well known substitute for glass such as polycarbonates, acrylicplastics, polyester resins and certain hard polyurethanes, with plies ofrelatively flexible interlayer material, such as plasticized polyvinylbutyral and polyurethanes, bonding the plies of rigid dielectricmaterial together to form a laminated window. In laminated glasswindows, the anti-static coatings are usually continuous transparentmetal or metal oxide coatings of low or moderate electroconductivitybonded to the outer glass sheet of the window. Laminated windowscontaining one or more glass sheets or other dielectric material layersin lieu of glass are provided with a transparent electroconductiveheating circuit.

In one type of heated laminated window, the heating circuit is carriedby the relatively flexible plastic interlayer sheet laminated betweenrigid transparent dielectric plies to form an electrically powerabletransparent laminated window. The electroconductive heating circuit ofthis type is either in the form of a transparent film carried on acarrier film embedded within an interlayer or a series of wires embeddeddirectly within the interlayer. Transparent electroconductive heatingcircuits and heating circuits composed of wire have also been applied onthe inner surface of outer plies of rigid transparent dielectricmaterials such as glass facing an adjacent flexible interlayer. Heatingcircuits containing electroconductive wire as the heating element eitherembedded in the flexible interlayer or adjacent to an interlayer surfacecould conduct only a limited amount of current before optical distortionresulted from a steep thermal gradient between the heating wire and thespace between adjacent wires. Therefore, wire embedded heating circuitsof the prior art had limited usefulness in aircraft windows.

In light planes where weight of windows is important, it is preferred tosubstitute, in place of glass, dielectric materials having less densitythan glass, such as plastics, for example, acrylics and polycarbonates,as the relatively rigid transparent dielectric layers of the laminatedwindow. However, transparent electroconductive coatings of metal ormetal oxide materials of suitable electroconductivity consistent withoptical transparency require relatively high temperatures for theirapplication to a substrate. Therefore, while it is suitable to applysuch coatings on glass substrates by pyrolysis of metal saltcompositions at high temperatures or by cathode sputtering, which isalso accomplished at elevated temperatures slightly less than thoserequired for pyrolysis, the high substrate temperatures required forpyrolysis or cathode sputtering make it impossible to obtainsatisfactory transparent electroconductive coatings on plasticsubstrates such as acrylics and polycarbonates without degrading thecomposition of the plastic substrate during the application of thetransparent electroconductive film. Furthermore, when the film isapplied at a temperature below which the plastic substrate tends todeteriorate, the film that forms has insufficient electroconductivity toprovide the heat needed for the purposes intended.

When an electroconductive heating circuit is applied to the inner majorsurface of the outer layer of a laminated window in the absence of aso-called anti-static circuit on the outward facing surface thereof, thecircuit provides a portion of an electrical path for grounding adischarge of static electricity that develops on the outer surface ofthe window when the latter is installed in a plane in flight. Since thecurrent path of the discharge is through the thickness of the outerlayer, a current discharge causes a hole through said entire thicknessof the outer layer, thus weakening the latter and eventually causing thepanel to fail in service.

In order to avoid damage to laminated windows resulting from thedischarge of static electricity through the thickness of the outerlayer, transparent electroconductive films produced by pyrolysis havebeen coated onto the outwardly facing surface of the outer glass layerof the laminated aircraft windows containing an outer glass layer. Whilefilms of suitable durability and electroconductivity can be formed onthe outer surface of glass sheets to enable the latter to dissipatestatic electricity before it discharges through the heating circuit fora reasonable period of service, the requirements for a low temperatureof application onto a plastic surface has made it difficult, if notimpossible, to obtain films to sufficient durability to serve asanti-static circuits when the latter are applied to the outwardly facingsurface of an outer layer of a laminated aircraft window when the outerlayer is composed of a plastic, such as acrylic plastic orpolycarbonate.

Prior to the present invention, none of the alternate transparentelectroconductive coatings available for use as anti-static coatings onplastic layers could provide the necessary combination of opticaltransparency, electroconductivity and durability in service for even asingle flight. Accordingly, attempts were made to develop a coating ofsufficient durability to last for a single flight so that each time aplane landed, the anti-static coating could be replenished. However, thelatter solution is not desirable because it requires additionaloperations each time a plane lands and, furthermore, no suitableanti-static coating has been developed which can be applied to a plasticsubstrate at temperatures normally encountered at airports.

2. Description of the Prior Art

From the foregoing, it is obvious that there is a need to overcome theseveral disadvantages of the laminated aircraft windows so as to providea durable static electricity dissipating circuit on the outer surface ofthe window, a heating circuit that does not develop poor opticalproperties, and suitable bonding between adjacent layers of thelaminated window. Also, a convenient method of producing a laminatedwindow with these improved characteristics is needed.

U.S. Pat. No. 2,813,960 to Egle et al shows a laminated heated window inwhich heating wires are sewn or embedded in an organic interlayermaterial such as cellulose derivatives, polyvinyl, polyamides orsilicones or in ceramic materials as well as glass so that thesematerials can be generally used for area heating either in transparentor opaque bodies. The heating element is completely embedded in theinsulating heated body material.

Other patents that use metal filaments or wire to carry electric currentin an electroconductive transparent panel include U.S. Pat. Nos.2,526,327 to Carlson; 2,932,710 to Coale et al; 3,484,583 and 3,484,584to Shaw; 3,729,616; 3,795,472; 3,745,309 and 3,895,433 to Gruss;3,414,713 to Reifeiss; 3,409,759 to Boicey et al; 3,223,829 to Davy etal; and 3,888,711 to Breitner. These latter references are directed toglass-plastic laminates in which electroconductive wires are embeddedwithin the interlayer of the laminate so that the heating wires aresupported within the plastic interlayer material.

Other laminated heated glass windows having electroconductive elementsapplied as transparent coatings to an inner surface of an outer glasssheet are found in many patents. U.S. Pat. No. 3,261,739 to Porter istypical.

In lightweight airplanes, the mass of laminated window units is animportant factor. Hence, it would be desirable to obtain heated windowsusing materials lighter than glass.

U.S. Pat. No. 3,310,458 to Mattimoe and Hofmann discloses a laminatecontaining an outer ply of a stretched acrylic resin, a vinyl butyralresin interlayer, and another layer of plastic having a continuouselectroconductive film facing the interlayer. The electroconductive filmis applied to the plastic layer other than the outer ply because of thedifficulty of applying a continuous electroconductive film on stretchedacrylic without harming its resistance to crack propagation. Such a unitis subject to destruction due to the build-up of static electricity onits exterior surface, which periodically discharges through the filmapplied to the other plastic layer. If a surface coating is applied tothe outer surface of an outer acrylic sheet at a substrate temperaturebelow that which would cause substrate damage, it must be applied byvacuum evaporation. However, such coatings are not permanent and theiranti-static properties are soon lost as the coating wears out.

Other heated laminated windows for aircraft used prior to the presentinvention found in many patents, of which U.S. Pat. No. 3,816,201 toArmstrong and Hoover is typical, comprise a pair of glass sheetslaminated together by a composite interlayer consisting essentially of acarrier layer of polyethylene terephthalate polyester film ("MYLAR")containing a gold electroconductive coating on one surface thereof andadhered by layers of polyvinyl butyral to outer sheets of glass. Thistype of system has several disadvantages. The laminated unit has atendency to delaminate at the surface between each interlayer and thegold film and also between the interlayer and the carrier layer.Furthermore, it is necessary to use in these units bus bar materials,which cure at low temperature and which are not very durable, incombination with the gold film that forms an electroconductive componentof the heating circuit.

U.S. Pat. No. 2,470,509 to Marini discloses the application of a pair ofheating wires disposed near the opposite surfaces of an interlayerdisposed between two glass sheets. The interlayer is press polishedbefore it is laminated to the glass sheets and the heating wires aredisposed at or near the interface of a relatively rigid transparentsheet of glass and a relatively flexible sheet of interlayer material.

U.S. Pat. No. 3,629,040 to Hinton et al discloses a method of applyingheating wires and feed conductors or bus bars directly to the surface ofa glass sheet, using an adhesive to maintain the wires in position andsubsequently applying electric current to burn off the adhesive. Asoldering iron is used to apply the bus bars to the heating wires.

SUMMARY OF THE INVENTION

An embodiment of this invention comprises a composite transparency orwindow comprising an outer ply of relatively hard transparent acrylicplastic having a first circuit comprising relatively widely spaced wiresembedded adjacent the outer major surface thereof and a second circuitcomprising relatively closely spaced wires embedded adjacent the innermajor surface thereof and an inner layer of hard transparent plasticselected from the class consisting of polycarbonates and acrylicplastics fused to said other major surface of said outer ply. Terminalmeans are connected to the first circuit and adapted for connection to aground to enable the first circuit to dissipate electrical charge thatdevelops on the outer surface of the window before the charge increasesto the point where it discharges at high voltage through the secondcircuit and damages the window, so that the first circuit behaves as ananti-static circuit when grounded. Additional terminal means adapted forconnection to a voltage source is connected to the second circuit toenable the second circuit to operate as a heating circuit when soconnected.

The inner layer of hard transparent plastic fused to the outer acryliclayer of the window insures that the second circuit is embedded inrelatively hard transparent plastic in spaced relation to any exposedsurface of a hard plastic layer. Thus, when the window is of thelaminated variety containing one or more layers of flexible interlayermaterial bonding the inner layer to one or more additional layers ofrigid dielectric material, the inner layer of rigid transparent plasticseparates the wires of the heating circuit from close adjacency to therelatively soft interlayer material. This separation of the heatingcircuit from the relatively soft interlayer material enables thelaminated window to have acceptable optical properties at significantlyhigher power densities resulting from voltage applied to the heatingcircuit compared to power densities that cause optical distortion whenapplied to heating circuits embedded in or adjacent the relatively softinterlayer.

The present invention provides a lightweight aircraft window having aheating circuit capable of being heated for defogging and defrostingpurposes and also provided with means to avoid degradation of the panelresulting from an accumulation of static electricity on the outersurface of the panel resulting in static discharge through the heatingcircuit that damages the panel and may even destroy the latter.

The present invention also provides heated plastic windows of suitablelightweight material to be used with lightweight aircraft thatincorporate therein a heating circuit providing good optical propertieseven when subjected to higher power densities that damage plasticwindows provided with transparent electroconductive coatings or withheating wire circuits disposed within a relatively low softening pointinterlayer material that also incorporate means in the form of ananti-static electroconductive wire circuit connected to an electrodethat is suitably grounded so as to dissipate any static electricitybefore the layer develops to the point where the window may be damagedby static discharge.

The present invention also provides a method of fabricating transparentwindows having relatively widely spaced wires embedded adjacent theouter surface of an outer layer of acrylic plastic and adapted to begrounded to dissipate a static electricity charge and insulated fromrelatively closely spaced embedded wires serving as an element of aheating circuit, which involves applying the embedded wire to a rigidtransparent dielectric layer other than the relatively soft interlayerand fabricating the laminated window in such a manner that in thelaminated window that results, the wires of the heating circuit lie insufficiently spaced relation to an interfacial surface with therelatively soft interlayer to minimize optical distortion of thelaminated window in use.

According to one embodiment of the method of the present invention, busbar elements are applied and wires are sewn into the opposite surfacesof a relatively rigid layer of acrylic plastic and the rigid layer ispress polished according to a cycle of steps to embed the wires ingrooves formed by sewing, to contact the wires with the bus bar elementsand to provide smooth surfaces and a thin film in the hard acrylic sheetto cover the wires and to help promote good optical properties in thelaminated window. The bus bars are connected to lead lines.

The wire sewn into one surface of said rigid layer is composed of widelyseparated runs, whereas the wire sewn into the other surface is composedof closely spaced runs. The first layer of rigid transparent plastic isthen assembled against a second layer of rigid transparent plastic freeof embedded wire with the surface containing the closely spaced wiresoriented to face the second layer. In such relationship, the presspolished sheet is then fused by heat and pressure to the second layer ofhard plastic to embed the heating circuit within the thickness of acomposite layer of hard transparent plastic that results from pressurefusing the layers.

In order to insure adequate electrode connection between the closelyspaced wires and its associated bus bars, the bus bars applied to thesurface containing the closely spaced wires are composed of two layers,one of which is embedded prior to sewing the closely spaced wires intothe substrate and the second layer of bus bars is applied after theheating wire is installed. The bus bars of both surfaces extend acrossthe wires near the ends of their elongated runs so as to make goodelectrical contact therewith.

Sewing is accomplished with minimum pressure necessary to embed thewires. Sewing produces grooves in the surface of the plastic. Sewingwith minimum pressure reduces the depth of the grooves formed by sewing.The wires are embedded and the grooves are smoothed by applying amoderate pressure while raising the temperature and, when the maximumtemperature of the press polishing cycle is attained, the plastic withthe wires in the grooves is subjected to pressure to embed the wires andbus bars within the smooth optical surface that results.

The composite layer can also be produced by an alternate method whichcomprises applying wire having widely spaced runs and its associated busbar arrangement to a surface portion of one layer, applying wire havingclosely spaced runs and its associated bus bar arrangement to a surfaceportion of a second layer, assembling the one layer with its surfacefree of wire facing the wire embedded surface of the second layer andfusing the layers together while so assembled to form a composite layer.The composite layer formed is suitable for use either as a window or asa transparency applied to the outer surface of a portion of a windowwhen produced by either of the alternate methods described.

If the resulting composite layer is to be laminated using typicalinterlayer material to bond the composite layer to one or more rigidtransparent layers, the final lamination is accomplished at a slightlylower temperature than the temperature used during the press polishingsteps that smooth the surfaces and embed the wire and bus bars in therigid transparent plastic composite layer. The wires, bus bars and leadwires of the anti-static circuit are spaced from those of the heatingcircuit for insulation purposes, thus allowing independent operation.

The present invention will be better understood in the light of adescription of a specific embodiment that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which form part of a description of an illustrativeembodiment of the present invention, and wherein like reference numbersrefer to like structural elements;

FIG. 1 is a fragmentary cross-sectional view taken along the lines I--Iof FIG. 15 of a corner portion of an assembly of a layer of acrylicplastic which forms the outer layer of a window conforming to thisinvention with one element of a compound bus bar applied against asurface thereof preparatory to a first press polishing step;

FIG. 2 is a fragmentary sectional view of the assembly portion of FIG.1, showing a pair of press polishing molds pressing said first bus barelement into the surface of the acrylic plastic layer;

FIG. 3 is a view similar to FIG. 1 showing the outer layer of the windowto be formed containing one element of the heating circuit bus barembedded in a press polished surface of the acrylic plastic layer afterthe method step depicted in FIG. 2 is completed;

FIG. 4 is a view similar to FIGS. 1 and 3 showing how the outer layerlooks after closely spaced runs of wire are embedded in grooves in onesurface of the acrylic layer with end portions of certain runs extendingbeyond said one bus bar element, widely spaced runs of wire are embeddedin the other surface of the acrylic layer, and a wire mesh type bus barapplied across an end portion of the widely spaced runs;

FIG. 5 is a view similar to FIG. 4 showing how a second bus bar elementis applied over the first bus bar element to sandwich an end portion ofthe runs of the closely spaced wires therebetween;

FIG. 6 is a fragmentary cross-sectional view similar to FIG. 2 showinghow the assembly of FIG. 5 is press polished between a pair of opposedpolishing molds;

FIG. 7 is a view similar to FIG. 5 showing how the respective bus barsand the wires are embedded within the opposite surfaces of the acryliclayer in the press polishing step of FIG. 6;

FIG. 8 is a fragmentary sectional view taken along the lines VIII--VIIIof FIG. 15 (generally at right angles to the view of FIG. 5) showing howthe wires applied to the opposite surfaces of the acrylic sheet aredeposited in grooves formed in the opposite surfaces of the acryliclayer;

FIG. 9 is a fragmentary view at right angles to that of FIG. 6 showinghow the acrylic sheet with the wires contained in the grooves formed inthe opposite surfaces is supported between a pair of press polishingmolds;

FIG. 10 is a view similar to FIG. 8 showing how the opposite wiresbecome embedded within the grooves in which they are deposited duringthe sewing operation and how the grooved surfaces become smooth duringthe press polishing step depicted in FIGS. 6 and 9;

FIG. 11 is a fragmentary schematic view showing how the press polishedacrylic layer of FIG. 10 is assembled with an additional layer of hardplastic between a pair of press polishing molds for a fusion operationto form a composite layer;

FIG. 12 is a view similar to FIG. 10 showing how the two acrylic layersthat are fused together as depicted in FIG. 11 appear as an integralcomposite layer after they are fused together;

FIG. 13 is a fragmentary view similar to FIG. 11 showing how thesubassembly of FIG. 12 is assembled with a layer of interlayer materialand another layer of rigid transparent material and interposed between apair of press polishing molds (only a fragment thereof being shown) forfurther lamination;

FIG. 14 is a fragmentary sectional view of a laminated window along thelines I--I of FIG. 15 resulting from the final lamination of theelements of the assembly as depicted in FIG. 13 and after the laminatedassembly is removed from between the press polishing members; and

FIG. 15 is a plan view of a window depicted either in FIG. 12 or FIG. 14and provided with a heating circuit formed from the closely spaced wiresand their associated two element bus bars and an anti-static circuitformed from the widely spaced wires and their associated wire mesh busbars according to the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawings depict a preferred embodiment of the invention and thevarious stages in a preferred fabrication method to fabricate saidpreferred embodiment. The latter comprises an outer layer 20 of acrylicplastic sheet, preferably polymethyl methacrylate sold under thetrademark PLEXIGLAS II by Rohm and Haas Company and conforming toMilitary Specification MIL-P-5425C. The outer layer 20 has opposed presspolished surfaces 24 and 32 and is fused to a second layer 50 of a rigidtransparent material, such as a polycarbonate resin or a second sheet ofacrylic plastic, which may be a second sheet of polymethyl methacrylateconforming to MIL-P-5425C or of a slightly higher cross-linkedpolymethyl methacrylate conforming to MIL-P-8184B sold under thetrademark PLEXIGLAS 55 or polycarbonate. If the second layer ispolycarbonate, it is preferably of the type described in U.S. Pat. No.3,388,032 to Saunders that is preferably prepared by reactingdi-(monohydroxyaryl)-alkanes with derivatives of carbonic acid such asphosgene and bischloro-carbonic acid esters ofdi-(monohydroxylaryl)-alkanes. The layers 20 and 50 form a compositelayer that can provide either a window or a transparency to be appliedto the outer surface of a portion of an installed window of an airplane.

The outer layer 20 of the window has an outer surface 32 that is presspolished and contains wire 30 sewn in the form of widely spaced,reversely connected elongated runs, the end portions at one side thereofbeing connected to an intermediate portion 37 of wire mesh 36 that isturned at right angles to form end portions 38. The latter are connectedto lead wires 39 which, in turn, are adapted for grounding connectionsto the frame of an aircraft in which the window or transparency isinstalled. The outer layer 20 has an inner surface 24 fused to a majorsurface 52 of the second hard plastic layer 50. The latter is providedwith opposite press polished surfaces 52 and 54. Along the inner surface24 are fused, reversely connected elongated runs of closely spaced wire40, some of which runs have end portions 42 sandwiched between an innerstrip 22 of tinned copper and an outer strip 34 of tinned coppersuperimposed in aligned relation over the inner strip 22 to form a pairof composite bus bars 22, 34 extending parallel to the end portions 38of mesh 36. Lead wires 44 are attached to corresponding ends of thecomposite bus bars 22, 34 and extend from the same side edge of thewindow as lead wires 39 in spaced relation thereto so that theanti-static circuit comprisig wire 30 is insulated from the heatingcircuit comprising wire 40.

The other major surface 54 of the second hard plastic layer 50 is alsopress polished and forms the outer surface of a composite layer thatforms a plastic window having an outer layer of plastic 20 having widelyseparated wires 30 of an anti-static circuit embedded near its outerexposed press polished surface 32 and closely spaced wires 40 of aheating circuit embedded adjacent its press polished surface 24 fused tothe press polished surface 52 of the second layer of rigid transparentplastic 50 with the second press polished surface 54 forming the exposedinner surface of the window. Such an embodiment is illustrated in FIG.12.

The composite layer of plastic of FIG. 12 may be further laminated to alayer 60 of interlayer material such as plasticized polyvinyl butyral orpolyurethane for adhering the composite layer 20, 50 to a portion of anouter surface of an aircraft windshield. A preferable plasticizedpolyvinyl butyral resin prepared as discloed in U.S. Pat. No. 2,400,957,although other polyvinyl acetal resins made from saturated orunsubstituted aliphatic aldehydes may be used. These would includepolyvinyl acetal resins made from unsubstituted saturated aliphaticcontaining less than six carbon atoms and especially those made fromformaldehyde, acetaldehyde, butyraldehyde and mixtures thereof.Paricularly preferred are polyvinyl acetal resins made frombutyraldehyde, the so-called polyvinyl butyral, preferably having amolecular weight ranging from 150,000 to 250,000. More details on thepreparation of polyvinyl acetal resins are found in U.S. Pat. Nos.Reissue 20,403 and 2,496,480.

Conventionally, such polyvinyl acetal interlayers contain a plasticizer,generally of a water-insoluble ester of a polybasic acid and apolyhydric alcohol. Particularly desirable plasticizers for use in thepresent invention are triethylene glycol di(2-ethyl butyrate), dibutylsebacate, di(beta-butoxy-ethyl) sebacate and dioctyl phthalate. Variousplasticizers for polyvinyl acetal resins are described in detail in U.S.Pat. No. 2,526,728 to Burk et al and in U.S. Pat. No. 2,372,522.

Polyurethanes are generally useful for interlayer material forinterlayer 60. A wide range of polyurethane compositions are useful,particularly polyester urethanes disclosed in the patent literature. Forexample, U.S. Pat. No. 3,931,113 to Seeger and Kaman discloses a familyof polyester polyurethane compositions having superior properties foruse in safety glass windshields that are formed from anhydroxy-terminated polyester of polycaprolactone or polybutylene adipateor polybutylene azelate or mixtures and a diol having a molecular weightless than 250, preferably 1,4-butanediol or 1,3-butanediol and acycloaliphatic isocyanate.

FIG. 14 shows a laminated window comprising a fused composite plasticlayer comprising an outer layer 20 of acrylic plastic supporting wiresembedded therein at different levels thereof and an inner layer 50 ofeither acrylic plastic or polycarbonate plastic laminated through alayer 60 of interlayer material in spaced relation to the wires embeddedin the outer layer 20 to an inner layer 70 of a rigid transparentmaterial such as glass or polycarbonate or acrylic plastic. It is alsounderstood that the inner layer 70 may be omitted in the final window,and also that additional alternate layers of interlayer material andrigid transparent material may be included in the final window.

It is also understood that various devices for attaching the laminatedwindow to an aircraft body well known in the art may be incorporated.Typical attachment mechanisms are disclosed in U.S. Pat. No. 3,919,022to Stefanik.

The following procedure represents an optimum method used to fabricateheated plastic windows for lightweight aircraft just described. In thefirst step, depicted in FIG. 1, a layer of acrylic plastic 20,preferably polymethyl methacrylate conforming to Military SpecificationMIL-P-5425C, which is available commercially as PLEXIGLAS II, is mountedin a horizontal position and a strip 22 of tinned copper is appliedagainst a first surface 24 of the acrylic plastic 20. The layer ofacrylic plastic 20 is then press polished between a pair of presspolishing molds of tempered glass 26 with a suitable parting material 28on the glass surfaces facing the acrylic plastic sheet. The presspolishing is performed at a preferred temperature of 300° to 315° F.(149° to 157° C.) and a pressure of 200 pounds per square inch (13.6atmospheres) for 60 to 90 minutes.

FIG. 2 shows the assembly as it is being press polished to embed onelayer of the tinned copper bus bar 22 into the surface 24 of the acrylicplastic sheet 20. FIG. 3 shows how the bus bar layer 22 is embedded intothe surface 24 of the acrylic sheet 20 as a result of the presspolishing operation of FIG. 2. A typical bus bar layer 22 is anelongated strip 5/16 inch (8 millimeters) wide, 2.5 mils (0.064millimeters) thick and the acrylic plastic layer has a thickness between60 and 125 mils (1.5 and 3.1 millimeters). The bus bar layer 22 ispositioned by applying a coating of an adhesive such as polyurethanedissolved in a suitable solvent (e.g., methyl ethyl ketone, chloroform,etc.) and tacking the lower bus bar layer 22 with a soldering iron tospaced points along surface 24.

The parting material 28 applied to glass press polishing molds 26 ispreferably of a material that adheres to glass and separates readilyfrom acrylic plastic and, preferably, also one that is readily separablefrom polycarbonate plastic. A good parting material that has beensuccessfully used is a coating of dimethyl dichlorosilane or the residuewhen dimethyl dichlorosilane is contacted with water such as moisture inthe atmosphere. However, other well known parting materials may besubstituted.

Wire for an anti-static circuit element is then sewn into surface 32 ofthe acrylic plastic sheet 20 which is opposite the first surface 24. Ina typical anti-static circuit, molybdenum-tungsten alloy wire having adiameter of 1.5 mil (0.04 millimeters) is applied in reversely arrangedruns spaced 2 to 4 inches (5 to 10 centimeters) apart and a wire meshscreen 36 is applied against the sewn wire in the plastic layer 20 nearthe corresponding ends of the runs of the wire 30. This sewing formswidely spaced, narrow grooves in the surface 32 having a preferredmaximum depth of 5 mils (0.13 millimeter). The wires 30 are destined tobecome elements of an anti-static circuit in the finished laminatedwindow.

Additional wire 40 destined to become a heating element of a heatingcircuit in the finished windshield and which is similar to the wires ofthe anti-static circuit except that the heating circuit wires areprovided in reversely arranged runs spaced from one another closertogether than the runs of wire 30, preferably approximately 30 mils(0.76 millimeter) apart, is then applied to the surface 24. Thisapplication is a form of sewing using a hot needle that forms a groovein the surface and is similar to the wire sewing of the anti-staticcircuit element. The depth of the grooves for the runs of wire 40 isapproximately the same as that for the runs of wire 30.

The runs of wire 40 extend along sinusoidal elongated lines that extendtransverse to the length of the strips of first bus bar layers 22. Theend portions 42 of certain periodic runs of heating wire 40 extendbeyond the first layer of tinned copper 22. A second layer 34 of tinnedcopper, also in the form of an elongated strip like layer 22 issuperimposed over each of the first layers 22 and tacked into positionthereover with a soldering iron. Strips 22 and 34 form composite busbars 22, 34 making electric connections with the end portions 42 ofcertain runs of heating wire 40 disposed therebetween. Lead wires 44 areconnected to the corresponding ends of the composite bus bars 22, 34 andare adapted for connection to a voltage source.

An elongated wire mesh 36 is disposed with its intermediate portion 37extending transverse to the length of bus bars 22, 34 and terminalportions 38 extending from the ends of intermediate portion 37 inapproximately parallel relation to the ends of composite bus bars 22,34. Lead wires 39 are connected to the terminal portions 38 and, whenthe window is installed in an airplane, are coupled to the airplaneframe to provide connections to ground. The intermediate portion 37 ofthe elongated wire mesh 36 extends parallel to the elongated runs of thewire 40 for the heating circuit and intersects the runs of the wire 30for the anti-static circuit to provide an electrical connection betweenthe runs of wire 30 and ground through portions 37 and 38 of wire mesh36, lead wires 39 and the airplane frame. However, there is noelectrical connection between the wire 30 for the anti-static circuitand the wire 40 for the heating circuit, because the respective wiresare applied to opposite major surfaces of the outer ply 20 and theirrespective leads are spaced from each other.

The assembly as depicted in FIG. 5 is then arranged between a pair oftempered glass plates 26 which serve as press polishing molds with arelease coating 28 similar to the one depicted in FIG. 2 facing theopposite surfaces of the acrylic layer 20 containing the closely spacedruns of the heating wire 40 embedded in grooves on its first surface 24thereof and the composite bus bar 22, 34 applied to said surface 24,while the relatively widely spaced anti-static wire 30 is embedded inthe second surface 32 thereof and the mesh 36 is applied to said secondsurface 32.

The assembly of glass and acrylic plastic sheets assembled as depictedin FIG. 6 is inserted in a laminating bag, preferably of the typedepicted in U.S. Pat. No. 3,255,567 of Keslar and Rankin. The laminatingbag (not shown) is formed of two plies including an outer ply ofpolyethylene glycol terephthalate, commercially known as Mylar, and aninner ply of polyethylene bound thereto. The inner ply of polyethyleneis embossed along its inner surfaces to form a checkerboard pattern ofrounded proturberances to permit the escape of air. The assembly,arranged as shown in FIGS. 6 and 9, is inserted within th bag, the bagis evacuated and sealed and the assembly subjected to the followingautoclave cycle. First, the assembly is heated to a temperature of 300°F. (149° C.) while maintaining a pressure of 20 to 25 pounds per squareinch (1.4 to 1.7 atmospheres) and held at this temperatrue and pressurefor 15 minutes. The pressure is then increased to 200 pounds per squareinch (13.6 atmospheres) and the temperature raised to 325° F. (163° C.)and held for 90 minutes. The temperature is reduced to 100° F. (38° C.),the pressure is lowered to atmospheric pressure, the bag is opened andthe assembly inspected.

FIGS. 7, 8 and 9 are fragmentary views taken at right angles to theviews of FIGS. 4, 5 and 6 showing how the acrylic plastic layer 20 isarranged with the wires 30 and 40 embedded in the opposite surfaces 24and 32. The inspection of the assembly reveals the surfaces 24 and 32press polished, with wires 30 and 40, mesh 36 and the first bus barlayer 22 embedded in the opposite surfaces and second layer 34 of thecomposite bus bar exposed.

The second layer of tinned copper 34 which was previously tacked inposition to the first bus bar layer 22 is removed and permanentlyattached to the first bus bar layer 27 by soldering. After the presspolishing of layer 20 is completed, and after the heating circuit busbar elements 22 and 34 are completely soldered to one another, the presspolished acrylic plastic layer 20 is mounted in such a position that itssurface 32 faces a glass mold 26 and its surface 24 faces a second layer50 of plastic which may be a second sheet of acrylic plastic or apolycarbonate sheet. The assembly of said plastic layers, one of whichis the acrylic plastic sheet 20 which has had its anti-static wire 30and heating wire 40 plus the bus bars embedded in the opposite surfacesthereof, is then assembled between a pair of press polishing molds 26having suitable parting material 28 as in the previously disclosedtempered glass molds and the subassembly so formed depicted in FIG. 11is autoclaved at a temperature of 315° to 325° F. (157° to 163° C.) anda pressure of 200 pounds per square inch (13.6 atmospheres) for 60 to 90minutes. This fuses the two plastic layers 20 and 50 together to form acomposite plastic layer depicted in FIG. 12.

While not shown, the composite plastic layer of FIG. 12 may befabricated by applying wire 30 and mesh 36 to surface 32 of layer 20 andwire 40 and composite bus bar 22, 34 to surface 52 of layer 50, presspolishing layers 20 and 50 separately after their respective treatmentsand assembling layer 20 against layer 50 with surface 24 facing surface52 for the fusion pressing step depicted in FIG. 11. For all practicalpurposes, the composite plastic layer 20, 50 that results from eithermethod of fabrication is suitable for use as a window or as atransparency to be applied to a portion of a surface of an aircraftwindow.

The composite plastic layer containing layer 20 and layer 50 fusedtogether is then assembled with a layer 60 of plastic interlayermaterial such as polyurethane or plasticized polyvinyl butyral and anadditional sheet of transparent material 70 assembled against theopposite surface of the layer 60, if further lamination is desired. Thesheet 70 may be of glass as well as polycarbonate or acrylic. The endportions 38 of the elongated wire mesh 36 serve as a bus bar for theanti-static wires 30 and may be soldered to ground leads 39 eitherbefore or after the final lamination. The final lamination is usuallyconducted at a temperature of 275° to 300° F. (135° to 149° C.) and at apressure of 200 pounds per square inch (13.6 atmospheres) for 60 to 90minutes. In all the laminating and press polishing operations, theassembly is inserted within a bag of the type depicted in the aforesaidKeslar and Rankin patent as in the first press polishing step.

Laminated windows produced by the method just described were tested incomparison with laminated assemblies that have heating wires embedded inan outer acrylic layer 20 in close adjacency to the interlayer 60. Thecomposite structures depicted in FIGS. 14 and 15 were able to withstandmuch higher power applied than the previous laminates having the heatingwires embedded at or near the interfacial surface between the interlayer60 and the outer acrylic sheet 20. The optics of the heated sheets wereimproved when the heating wires were separated from the plasticinterlayer by the thickness of the second plastic layer 50 (of eitheracrylic or polycarbonate) compared to when the heating wires werelocated at or near the interfacial surface between the layer 50 and apolyurethane interlayer 60.

A preferred embodiment of this invention consists of an outer layer ofPLEXIGLAS II (polymethyl methacrylate conforming to MilitarySpecification MIL-P-5425) 60 mils (1.5 millimeter) thick fused to aninner layer of PLEXIGLAS 55 (polymethyl methacrylate sheet conforming toMilitary Specification MIL-P-8184) also 60 mils (1.5 millimeter) thick,an interlayer of polyurethane 125 mils (3.1 millimeters) thick and aninner layer of polycarbonate 0.125 inch (3.1 millimeters) thick. Theembedded wires for both the heating and the anti-static circuits were ofmolybdenum-tungsten wire 1.5 mil (0.038 millimeter) thick with itsreversely connected runs spaced 2 inches (5 centimeters) apart from runto run in the anti-static circuit and 30 mils (0.76 millimeter) apart inthe heating circuit. The bus bar for the anti-static circuit was a wiremesh 0.0055 inch (0.14 millimeter) thick, approximately 174 inch (6.35millimeters) wide and the bus bars for the heating circuit were composedof two layers of 2.5 mil (0.064 millimeter) thick tinned copperapproximately 1/4 to 3/8 inch (6.35 to 9.52 millimeters) widesuperimposed on one another against opposite surfaces of the embeddedwire runs forming the heating circuit.

The present invention has been tested and compared with prior art heatedwindows of various types. When compared with laminated windows usinggold films carried on Mylar and laminated to outer plies of rigidtransparent material, the present structure has been found superior.

Gold films with adequate transparency have an upper limit ofelectroconductivity such that their surface resistivity is limited toless than 40 ohms per square. The gold film, in order to develop alesser electrical resistivity, must be made so thick that its opticalproperties are impaired, as is its ability to adhere to the carrier filmon which it is deposited. Hence, this type of heated laminated windowimposes severe design limitations compared to the capabilities of thepresent invention. Therefore, a heating circuit containing thinelongated wires embedded in plastic has proven to be an extremelyreliable system for laminated transparencies and can be designed to amuch lower electrical resistance compared to that with which gold filmsapplied to carrier films sandwiched between layers of interlayermaterial are practical to use.

Heating wire circuits used in the interlayers at or near theglass-plastic interfaces of laminated glass transparencies haveacceptable optics when the heating circuits are used in power densitiesas high as 3.5 watts per square inch (2.26 kilowatts per square meter).However, when heating wire is sewn in a thermoplastic interlayer such asthe plasticized polyvinyl butyral normally used in laminated heatingunits whose outer layers are composed of acrylic or polycarbonateplastics, power densities of as little as 1.5 to 2 watts per square inch(1 to 1.3 kilowatts per square meter) cause the optical properties todegenerate to the point where they become unacceptable. This powerdensity if not adequate for deicing requirements which require powerdensity of 4.5 watts per square inch (3 kilowatts per square meter) incertain cases. The present invention has determined that separating thewire of the heating circuit from the interlayer material improves theresistance of the laminated window to optical distortion when theheating circuit is energized.

When a heating circuit embedded in a polyvinyl butyral interlayer isheated for defogging and deicing purposes, the heated area in thevicinity of the heating wires become fuzzy. This fuzziness is believedto be due to the thermal gradient that is created in the interlayerbetween the adjacent hot wires. Since the thermoplastic interlayers arevery poor thermal conductors, the interlayer becomes very hot near thewires and a steep thermal gradient develops in the interlayer to aminimum temperature at the mid-point between adjacent wires when theheating wire is energized. This thermal gradient is believed to cause agradient in index of refraction within the interlayer which gives anappearance of fuzziness.

Plastic laminates containing hard rigid transparent sheets of eitheracrylic plastic or polycarbonate laminated with layers of interlayermaterial having electroconductive heating wires embedded therein provideeven worse optical properties than corresponding laminates of theaforesaid thermoplastic interlayer materials to glass. The reason forthis deterioration of optical properties is believed to be because glassis a better thermal conductor (approximately four times better) thaneither acrylic or polycarbonate plastic. Consequently, the thermalgradient and index of refraction gradient are more severe in thelaminated plastic windows than in the laminated glass windows. However,the interposition of a second rigid plastic layer 50 between the heatingwire 40 and the plastic interlayer 60 provides a structure that reducesthe tendency for optical distortion to result at given power densities.A minimum thickness of at least half the wire spacing for the secondlayer is usually sufficient to improve the optical properties of thelaminated window, because such a layer separates the spaced heating wire40 from the relatively soft interlayer by a sufficiently thick layer ofrelatively hard plastic to enable the laminated transparency or windowso formed to have the heat radiated or conducted from adjacent runs ofheating wire blend to a more uniform heat pattern when the heatradiation reaches the soft interlayer.

Having a heating circuit embedded within the thickness of a fused outercomposite layer of relatively rigid transparent plastic also wouldprovide a circuit for discharge of static electricity through thethickness of the rigid plastic with consequent damage and evendestruction to the rigid transparent plastic. Since the continuousanti-static coatings applicable to glass are not sufficiently durablefor plastic surfaces, the use of a wire circuit embedded within andadjacent the outer surface of the window or transparency provides acharge dissipating circuit that inhibits the development of a highvoltage charge that tends to discharge through the thickness of theouter layer and the wire of the heating circuit.

While the specific embodiments just described comprise both a heatingcircuit element and a static electricity dissipation (or "anti-static"0circuit element, both comprising electroconductive wire embedded inrigid transparent plastic, the term "rigid" signifying a materialsignificantly less flexible than conventional interlayer material usedto bond relatively rigid layers together, and rigid materials areexemplified by polycarbonate and acrylic plastics recited herein, webelieve that the present invention also provides a novel staticdissipation circuit comprising electroconductive wire embedded adjacentan exposed surface of a hard plastic transparency or window, preferablyat a depth no greater than 5 mils (0.13 millimeter) and adapted forconnection to ground with or without a heating circuit.

In the claims that follow, the term "terminal means" is used to refer tothe composite bus bars 22, 34 of the heating circuit and the wire mesh36 of the static electricity discharge (or anti-static) circuit.

The form of the invention described herein is a preferred embodiment ofthe present invention and certain modifications thereof. For example,other bus bar materials may be substituted for the mesh 36, such asconductive metal strips, other heating wire such as tungsten or othermetal wire may be substituted, and the outer layer 20 having the wire 30of the anti-static circuit embedded therein may be composed of anysuitable rigid transparent plastic, such as polycarbonate resin, or evena rigid hard transparent polyurethane composition, the interlayer 60 maybe composed of any other flexible interlayer material such as siliconeresins and composites of layers of the same or dissimilar flexibleinterlayer materials and many other changes may be made in the methoddescribed such as eliminating the press polishing step of FIGS. 6 and 9in order to reduce the possibility of breaking wire during thefabrication of the composite transparency of FIG. 12. It is understood,however, that other changes may be made without departing from the gistof the present invention as recited in the claimed subject matter thatfollows.

We claim:
 1. A method of making a composite transparency comprisingapplying a pattern of wire to an opposite pair of major surfaces of afirst layer of rigid transparent plastic selected from the classconsisting of acrylic plastic and polycarbonates, one of said patternscomprising relatively widely spaced runs of continuous wire and theother of said patterns comprising relatively closely spaced runs ofcontinuous wire, press polishing said layer of rigid transparent plasticat sufficient heat and pressure to smooth the opposite major surfacesthereof so that the wire is embedded just below each of the oppositemajor surfaces, attaching suitable terminal means to the wire embeddedadjacent each of the opposite surfaces, and fusing a second layer ofrigid transparent plastic selected from the class consisting of acrylicplastic and polycarbonates at elevated pressure and temperature againstthe first layer of plastic having the wires embedded therein to form acomposite transparency having wire with widely spaced runs embeddedadjacent a major surface thereof and wire with closely spaced runsembedded within the thickness in relatively spaced relation from theopposite major surfaces of side composite transparency.
 2. A method asin claim 1, further including laminating a layer of plastic interlayermaterial taken from the class consisting of polyurethanes andplasticized polyvinyl acetals to the major surface of said compositetransparency that is spaced from both of said wire runs.
 3. The methodas in claim 2, comprising laminating a rigid transparent layer takenfrom the class consisting of glass, polycarbonate plastic, and acrylicplastic to the opposite surface of said layer of interlayer materiallaminated to said composite transparency.
 4. A method as in claim 1,comprising applying a first pair of strips of electroconductive materialto said surface to which said closely spaced runs of wire are applied,applying said closely spaced runs so that the end portions of at leastseveral of said runs extend beyond said first layers ofelectroconductive material, applying a second strip of electroconductivematerial in superimposed relation over each of said first strips withsaid end portions disposed between said first and second strips ofelectroconductive material in strip form prior to said press polishing.5. A method as in claim 1, comprising applying elongated wire mesh tothe surface containing the relatively widely spaced runs of wire incontacting relation to an end portion of at least several of said runsprior to said press polishing step.
 6. A method as in claim 1, furthercomprising applying a first pair of strips of electroconductive materialto said surface to which said closely spaced runs of wire are applied,applying said closely spaced runs so that the end portions of at leastseveral of said runs extend beyond said first strips ofelectroconductive material, applying a second strip of electroconductivematerial in superimposed relation over each of said first strips withsaid wire end portions disposed between said first and second strips ofelectroconductive material prior to said press polishing, applyingelongated wire mesh to the surface containing the relatively widelyspaced runs of wire prior to said press polishing step, attachingterminal means to said composite strips of electroconductive materialthat is bonded to said relatively closely spaced runs of said one wire,and connecting additional terminal means to the elongated wire mesh inelectroconductive contact with said relatively widely spaced runs ofsaid wire.
 7. A method of making a composite transparency comprisingapplying a pattern of widely spaced wire runs to a major surface of afirst layer of rigid transparent acrylic plastic, applying a pattern ofclosely spaced wire runs to a major surface of a second layer of rigidtransparent plastic selected from the class consisting of acrylicplastic and polycarbonates, attaching suitable terminal means to endportions of at least several of each of said runs, press polishing saidfirst and second layers to embed said wire runs within and just belowsaid respective major surfaces, assembling said layers with said majorsurface of said first layer facing away from said second layer and saidmajor surface of said second layer facing said first layer, and fusingsaid second layer while so assembled against the first layer of plastichaving the wires embedded therein to form a composite transparencyhaving wire with widely spaced runs embedded adjacent a major surfacethereof and wire with closely spaced runs embedded within the thicknessin relatively spaced relation from the opposite major surfaces of saidcomposite transparency.
 8. A method as in claim 7, further includinglaminating a layer of plastic interlayer material taken from the classconsisting of polyurethanes and plasticized polyvinyl acetals to themajor surface of said composite transparency that is spaced from both ofsaid wire runs.
 9. The method as in claim 8, comprising laminating arigid transparent layer taken from the class consisting of glass,polycarbonate plastic, and acrylic plastic to the opposite surface ofsaid layer of interlayer material from that laminated to said compositetransparency.
 10. A method as in claim 7, comprising applying a firstpair of strips of electroconductive material to said surface to whichsaid closely spaced runs of wire are applied, applying said closelyspaced runs so that the end portions of at least several of said runsextend beyond said first layers of electroconductive material, applyinga second strip of electroconductive material in superimposed relationover each of said first strips with said end portions disposed betweensaid first and second strips of electroconductive material in strip formprior to said press polishing.
 11. A method as in claim 7, comprisingapplying elongated wire mesh to the surface containing the relativelywidely spaced runs of wire in contacting relation to an end portion ofat least several of said runs prior to said press polishing step.
 12. Amethod as in claim 7, further comprising applying a first pair of stripsof electroconductive material to said surface to which said closelyspaced runs of wire are applied, applying said closely spaced runs sothat the end portions of at least several of said runs extend beyondsaid first strips of electroconductive material, applying a second stripof electroconductive material in superimposed relation over each of saidfirst strips with said wire end portions disposed between said first andsecond strips of electroconductive material prior to said presspolishing, applying elongated wire mesh to the surface containing therelatively widely spaced runs of wire prior to said press polishingstep, attaching terminal means to said composite strips ofelectroconductive material that is bonded to said relatively closelyspaced runs of said one wire, and connecting additional terminal meansto the elongated wire mesh in electroconductive contact with saidrelatively widely spaced runs of said wire.
 13. A method of making acomposite transparency comprising applying a pattern of wire to anopposite pair of major surface of a first layer of rigid transparentplastic selected from the class consisting of acrylic plastic andpolycarbonates, one of said patterns comprising relatively widely spacedruns of continuous wire and the other of said patterns comprisingrelatively closely spaced runs of continuous wire, attaching suitableterminal means to the wire adjacent each of the opposite major surfaces,assembling a second layer of rigid transparent plastic selected from theclass consisting of acrylic plastic and polycarbonates against thesurface of said first layer to which said closely spaced runs areapplied, and press polishing said assembly at elevated pressure andtemperature to fuse said second layer to the first layer of plastic andto embed the wires therein to form a composite transparency having wirewith widely spaced runs embedded adjacent and just below a major outersurface of said transparency and wire with closely spaced runs embeddedwithin the thickness in relatively spaced relation from the opposite,press polished major surfaces of said composite transparency.